Immersible PTC heating device

ABSTRACT

An electrical heating device ( 10 ) for heating liquids employing one or more positive temperature coefficient (PTC) thermistors as heating elements ( 11 ). These heating elements ( 11 ) are in direct thermal and electrical contact on opposing sides ( 22 ), which are coated with a conductive metal such as aluminum, with electrodes to supply electrical current. These electrodes are in turn in direct thermal contact on their outward-facing sides with respect to the heating elements with plates ( 15 ) which are formed of a thermally conductive and electrically insulating ceramic material such as AL203, and the plates are in direct thermal contact on their outward-facing sides with respect to the heating elements with heat radiation units ( 20 ) which include cooling fins for heat transfer. The heating elements are positioned by an electrically and thermally insulating frame made of heat-resistant material which serves, together with the radiation units, to fully enclose the heating elements, thereby preventing liquid from entering the interior of the device, thus rendering it immersible.

FIELD OF THE INVENTION

The present invention relates to electrical heating devices for heatingliquids, particularly those employing thermistors with positivetemperature coefficient of resistance (PTC) as heating elements.

BACKGROUND OF THE INVENTION

Positive temperature coefficient (PTC) heating elements, such asthermistors, are used in electrical heating devices, such as electricalradiators, electrical heating fans, and air conditioner heaters. Theyhave an advantage over electric wire heaters in that they areself-regulating as to temperature and thus are not subject tooverheating even in response to abnormal electric currents. In manyprior art applications employing PTC thermistor heating elements, heatis extracted from the device by air flow through the device, includingthe heating elements and radiating elements, such as radiating fins. PTCheating elements, however, are very sensitive to temperature variations,even those caused by air flow across them, due to the “pinch effect”(current displacement), which causes reduced heating efficiency andgenerating power and reduces their lifetime. As will be appreciated bypersons skilled in the art, this problem is more severe for applicationswherein the PTC device is used to heat a liquid.

U.S. Pat. Nos. 5,598,502 and 5,471,034 disclose PTC thermistor devicesfor heating liquids wherein the PTC heating elements are enclosed. Inboth these cases, the devices disclosed have thermal resistance betweenthe PTC elements and heat transfer members that reduces their heattransfer efficiency and, thus, their power output. The former device hasthe additional disadvantage of not being fully immersible. The latterdevice employs channels for the liquid through heat transfer members,which complicates their construction and integration into liquid systemsto be heated.

SUMMARY OF THE INVENTION

The present invention seeks to provide an electrical heating device forheating liquids employing positive temperature coefficient (PTC)thermistors as heating elements, which overcomes disadvantages of knownart by providing a total enclosure for the PTC heating elements and lowthermal resistance between the heating elements and the radiatorelements. These features allow a heating device that is totallyimmersible with efficient internal heat transfer, high power output, andlong life.

There is thus provided, in accordance with a preferred embodiment of theinvention, an electrical heating device for heating liquids employingone or more positive temperature coefficient (PTC) thermistors asheating elements. These heating elements are in direct thermal andelectrical contact on opposing sides, which are coated with a conductivemetal such as aluminum, with electrodes to supply electrical current.These electrodes are, in turn, in direct thermal contact on theiroutward-facing sides with respect to the heating elements with plateswhich are formed of a thermally conductive and electrically insulatingceramic material such as AL₂O₃, and the plates are in direct thermalcontact on their outward-facing sides with respect to the heatingelements with heat radiation units which include cooling fins for heattransfer. The heating elements are positioned by an electrically andthermally insulating frame made of heat-resistant material which serves,together with the radiation units, to fully enclose the heatingelements, thereby preventing liquid from entering the interior of thedevice, thus rendering it immersible.

The internal elements of the heating device are further held in placeand in good thermal contact with each other by mechanical pressure andby a thermally and electrically conductive adhesive. The insulatingplates are coated on their inward-facing with respect to the heatingelements with a conductive metal such as aluminum to provide goodthermal contact therewith. The conductive coatings on the insulatingplates can serve as the electrodes for the device, or the electrodes canoptionally be independent elements with a textured surface to providegood thermal contact with adjacent members of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood and appreciated fromthe following detailed description, taken in conjunction with thedrawings, in which:

FIG. 1A is a schematic side-sectional view of an electrical heatingdevice constructed and operative in accordance with a preferredembodiment of the present invention;

FIG. 1B is a cross-sectional view of the electrical heating device ofFIG. 1A, taken along line S—S therein;

FIG. 1C is a cross-sectional view of the electrical heating device ofFIG. 1A, taken along line S—S therein, constructed and operative inaccordance with an alternative preferred embodiment of the presentinvention;

FIG. 2A is a side-sectional view of the electrical heating device ofFIG. 1A, taken along line Q—Q therein;

FIG. 2B is a side-sectional view of the electrical heating device ofFIG. 1A, taken along line R—R therein;

FIG. 3 is a front view of the electrical heating device of FIG. 1A,taken in the direction of arrow P therein.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1A through 3, there is shown an electricalheating device referred to generally as 10, constructed in accordancewith a preferred embodiment of the present invention. Electrical heatingdevice 10 has an array of one or more heating elements 11 which arepositive temperature coefficient (PTC) thermistors. They are fabricatedwith preferably parallel, generally flat, surfaces on opposing faces 22,which are metallized, preferably by provision of a coating of aconductive metal such as aluminum, to serve as thermal and electricalcontact surfaces. On opposing sides of heating elements 11 are plates 15which are formed of a thermally conductive and electrically insulatingceramic material such as AL₂O₃. Inward-facing surfaces 16 of insulatingplates 15 are coated with a conductive metal such as aluminum, therebyconstituting thermal and electrical contact surfaces.

In a preferred embodiment of the present invention, shown in FIG. 1B,these coated surfaces 16 of insulating plates 15 serve as electrodes forheating device 10, to supply electrical current to heating elements 11thereby.

In an alternative preferred embodiment of the present invention, shownin FIG. 1C, there are provided electrodes 19 which are disposed betweeninsulating plates 15 and PTC heating elements 11. Electrodes 19, which,preferably, are provided in conjunction with each of coated surfaces 16(FIG. 1A), are operative to supply electrical current to heatingelements 11. In this embodiment, PTC heating elements 11 are in thermaland electrical contact with electrodes 19 via their metallized surfaces22 (FIG. 1A). Electrodes 19 are fabricated to have spring-likeproperties and textured surfaces 21 and 23. Preferably, electrodes 19are formed of a corrugated springy conductive sheet metal, preferably,aluminum. As will be appreciated by persons skilled in the art, theseproperties provide especially good mechanical, thermal, and electricalcontact, both between electrodes 19 and heating elements 11 and alsobetween electrodes 19 and metallized surfaces 16 of insulating plates15. The present embodiment is operative to supply generally higherthermal energy output levels than the embodiment shown in FIG. 1B.

Referring now to FIG. 1A, disposed on outward-facing surfaces 26 ofinsulating plates 15 and in direct thermal contact therewith are heatradiator units, referred to generally as 20, each of which includes aplate 12 and cooling fins 13 extending generally transversely therefrom.Radiator units 20 are made of a material that is a good thermal andelectrical conductor, such as aluminum or copper. The plates 12 of theradiator units 20 are fabricated with flat inward-facing surfaces 18 toserve as thermal contact surfaces. The radiator plates 12 are positionedso that the inward-facing contact surfaces are generally parallel to andin touching contact with the outward-facing contact surfaces 26 ofinsulating plates 15 so as to define thermal interfaces therewith. Theconduction across the interfaces is improved by the use thereat of asuitable thermally and electrically conductive adhesive. An example ofone material that is adequate for this purpose is Ceramabond™ TM5526, ahigh-temperature adhesive produced by Aremco Products, Inc. of Ossining,N.Y. 10562, U.S.A.

In accordance with a preferred embodiment of the invention, the adhesiveis the novel adhesive developed by the present inventors, disclosed inIsrael Patent Application No. 121449. More particularly, the adhesive isan electrically and thermally conductive adhesive composition, in whichthe adhesive component is essentially a curable silicone prepolymer, andthe composition includes finely divided silicon carbide withfinely-divided silicon, either separately or in admixture.

The composition is preferably additionally characterized by at least oneof the following features, namely:

(i) the finely-divided metallic powder has a particle size no greaterthan about 40 μm;

(ii) said finely divided silicon carbide and finely divided silicon haveparticle sizes no greater than about 14 μm;

(iii) said finely divided silicon carbide and finely-divided silicon arepresent in a respective weight ratio of about 0.9 to 1.1: about 1.0;

(iv) the respective weight ratios of said finely-divided metallicpowder, said finely divided silicon carbide taken together with finelydivided silicon, and said curable silicone prepolymer, are 0.1 (±5%):1.1(±5%):1 (±5%), and preferably 0.1 (±1%):1.1 (±1%):1 (±1%).

It is also a particularly preferred feature of the present adhesivecomposition, that the curable silicone prepolymer has a viscosity atambient temperature within the range 15,000 to 25,000 μPa/sec.

PTC thermistor heating elements 11 convert electrical energy appliedthereto to thermal energy. The thermal energy is, in turn, conductedfrom heating elements 11 to cooling fins 13 via electrodes 19, wherepresent, insulating plates 15, all thermal interfaces, and radiatorplates 12. When heating device 10 is immersed in a liquid, the thermalenergy flows from fins 13 to the liquid.

Referring now to FIGS. 2A and 2B, there are shown two side-sectionalviews of electrical heating device 10 as shown in FIG. 1A, taken alonglines Q—Q and R—R therein, respectively. An array of PTC heatingelements 11 are positioned by an electrically and thermally insulatingframe, referenced generally as 14, made of heat-resistant material. Asshown in FIG. 1A, frame 14 has flanges 14A on either side of heatingelements 11, and end pieces 14B and a conduit 14C for heat-resistantwires 28 to supply electrical current to heating device 10. The sectionpictured in FIG. 2B is through insulating plate 15 which is inelectrical and thermal contact with heating elements 11 at contactsurface 22 on one side thereof, as shown in FIG. 1A.

It can be seen from FIG. 2A that flanges 14A and end pieces 14B andconduit 14C of positioning frame 14 surround the array of heatingelements 11 on four sides. Referring again to FIG. 1A, the top flange14B of positioning frame 14, shown partially cut away, can be seen toenclose the array of heating elements 11 from above, as drawn, andplates 12 of radiator units 20 can be seen to enclose heating elements11 on both sides longitudinally, as drawn. Plates 12 of radiator units20 are joined to end pieces 14A of insulating frame 14 and their ends12A to complete the enclosure of heating elements 11 and of the interiorof heating device 10.

In these cross-sectional views, the array of heating elements 11 and theinterior of heating device 10 is seen to be completely enclosed by frame14 and radiator unit plates 12, thereby preventing fluid from enteringthe interior of heating device 10, thus rendering it immersible.

Referring now to FIG. 3, there is shown a front view of an electricalheating device 10 constructed in accordance with a preferred embodimentof the present invention. In this view is shown one of the radiatorunits with its plate 12 and fins 13. The drawing also shows conduit 14Cof positioning frame 14 with two heat-resistant wires 28 coming outthereof. Heating device 10 as shown is immersible up to the top ofconduit 14C of positioning frame 14 which can optionally be constructedwith a water-tight seal around heat-resistant wires 28 or packed with asuitable water-tight and heat resistant sealant thereby rendering thedevice totally immersible.

It will further be appreciated, by persons skilled in the art that thescope of the present invention is not limited by what has beenspecifically shown and described hereinabove, merely by way of example.Rather, the scope of the present invention is defined solely by theclaims, which follow.

I claim:
 1. An immersible, electrical device for heating liquids, whichcomprises: at least one positive temperature coefficient (PTC)thermistor heating element having generally parallel, flat, contactsurfaces; a pair of plate members formed of an electrically insulatingand thermally conductive material having generally parallel, flat,inward- and outward-facing contact surfaces and wherein saidinward-facing contact surfaces are coated with a thermally andelectrically conductive metal; a pair of heat radiation members formedof an electrically and thermally conductive material, each comprising: aplate portion having a generally flat, inward-facing, contact surfacefor thermally conductive contact with said outward-facing contactsurfaces of said plate members and a plurality of generally outwardlyextending cooling fins formed on said plate portion; electrode meansabout said at least one heating element operative to provide thermallyand electrically conductive contact with said contact surfaces thereofand thermally conductive contact with said inward-facing contactsurfaces of said plate members; means for fastening said heat radiationmembers about said plate members such that said inward-facing, contactsurfaces of said plate portions are held in thermally conductive contactwith said outward-facing contact surfaces of said plate members therebyto define therewith thermal interfaces, said means for fastening beingfurther operative to fasten said plate members about said electrodemeans and said electrode means about said at least one heating elementsuch that said inward-facing, contact surfaces of said plate members areheld in electrically and thermally conductive contact with saidelectrode means and said electrode means are held in electrically andthermally conductive contact with said flat contact surfaces of said atleast one heating element thereby to define therewith thermo-electricinterfaces; enclosing means formed of an electrically and thermallyinsulating material and formed for positioning around said at least oneheating element and between said plate portions of said heat radiationmembers so as to prevent any liquid from coming into contact with theinterior of said heating device; and terminal means attached to saidelectrode means, operative to permit flow of electric currenttherethrough, across said interfaces, and via said at least one heatingelement, thereby producing thermal energy therein and operative toprevent any liquid from coming into contact with the interior of saidheating device; wherein, when an electrical current passes through saidelectrical heating device, thermal energy from said at least one heatingelement is conducted across said interfaces to said heat radiationmembers.
 2. An electrical heating device according to claim 1 whereinsaid contact surfaces of said at least one heating element are coatedwith a thermally and electrically conductive metal.
 3. An electricalheating device according to claim 1 wherein said electrode meanscomprises said metal-coated inward-facing surfaces of said platemembers.
 4. An electrical heating device according to claim 1 whereinsaid electrode means comprises a thermally and electrically conductivemetallic sheet with a multiplicity of protuberances to providespring-like properties thereto.
 5. An electrical heating deviceaccording to claim 1 wherein said means for fastening comprisescompression means.
 6. An electrical heating device according to claim 1wherein said means for fastening comprises an adhesive which iselectrically and thermally conductive applied to all said contactsurfaces of all said interfaces.
 7. An electrical heating deviceaccording to claim 1 wherein said terminal means comprisesheat-resistant wires.
 8. An electrical heating device according to claim1 wherein said enclosing means comprises a positioning frame and a pairof flanges.
 9. An electrical heating device according to claim 8 whereinsaid at least one heating element comprises at least two heatingelements and said enclosing means further comprises spacing membersoperative to position said at least two heating elements so as toprevent touching contact therebetween.